Acetylene blockage was evaluated as a method for measuring losses of N 2 0 + N, from two Denchworth series clay soils. The denitrification potential in anaerobic, dark incubations at 20°C with nitrate (equivalent to 100 kg N ha-' 0-20 cm depth), maximum water holding capacity, and acetylene ( IYo), was equivalent to 32 _+ 1 1 and 39 k 6 kg N ha-' per day for the two 0-20 cm soils and was positively correlated with carbon content (r=0.98). After 4 days N20 was reduced to Nz in the presence of C2H,.In April 1980 following irrigation (24 mm) and applications ofammonium nitrate (70 kg N ha-') and acetylene, the mean nitrous oxide flux from soil under permanent grass was 0.05f0.01 kg N2O-N ha-' per day for 8 days.In June 1980, the losses of nitrogen from cultivated soils under winter wheat after irrigation (36 mm) and acetylene treatment were 0.006 f0.002 and 0.04-0.07+0.0I kg N ha-' per day respectively before and after fertilizer application (70 kg N ha-'). The nitrous oxide flux in the presence ofacetylene decreased briefly, indicating that nitrification was rate determining in drying soil.

Calcium nitrate fertilizer containing 92.3 atoms O/ o excess nitrogen-I 5 was applied on 5 May 198 1 at a rate equivalent to 100 kg N ha-' to a clay soil in southern England cropped to winter wheat. Samples of the soil gases were collected frequently during the following 3 weeks. The soil oxygen concentration declined to 5% after 60 mm rain. A maximum of 1.5 k0.5 atom % N-15 enrichment in labelled N, gas (29N,) was detected in the soil atmosphere on 28 May. Total denitrification losses, calculated from air-filled pore space and rates of gas loss from the soil estimated using a Fick's law approximation, were 9.5 kg N ha-' with a daily rate of 0.30-tO.07 kg N ha-'. Estimated total losses were greater than 30 kg N ha-', 93% in the form N,, but the estimation depends on several assumptions about the amount of double labelled gas (' ON,), rates of gas diffusion and flux.

SUMMARY Emissions of nitrous oxide (N2O) and nitrogen gas (N2) from denitrification were measured using the acetylene inhibition method on drained and undrained clay soil during November 1980‐June 1981. Drainage limited denitrification to about 65% of losses from undrained soil. Emissions from the undrained soil were in the range 1 to 12 g N ha–1 h–1 while those from the drained soil ranged from 0.5 to 6 g N ha–1 h–1 giving estimated total losses (N2O + N2) of 14 and 9 kgN ha–1. Drainage also changed the fraction of nitrous oxide in the total denitrification product. During December, emissions from the drained soil (1.8±0.6 gN ha–1 h–1) were composed entirely of nitrous oxide, but losses from the undrained soil (2.7 ± 1.1 g N ha–1 h–1) were almost entirely in the form of nitrogen gas (the fraction of N2O in the total loss was 0.02). In February denitrification declined in colder conditions and the emission of nitrous oxide from drained soil declined relative to nitrogen gas so that the fraction of N2O was 0.03 on both drainage treatments. The delayed onset of N2O reduction in the drained soil was related to oxygen and nitrate concentrations. Fertilizer applications in the spring gave rise to maximum rates of emission (5–12g N ha–1 h–1) with the balance shifting towards nitrous oxide production, so that the fraction of N2O was 0.2–0.8 in April and May.